1. Field of the Invention
The present invention relates to a treatment planning system. More particularly, the invention relates to a treatment planning system for use with radiation therapy equipment that applies ion beams such as X-rays and proton beams to the tumor for treatment.
2. Description of the Related Art
In recent years, radiation therapy aimed at applying various types of radiation to tumor cells to induce their necrosis has been practiced extensively. The radiation in use is not limited to the most commonly used X-rays; particle beams such as proton beams are gaining widespread acceptance for treatment.
The irradiation technology for handling radiation has been highly developed. Progress has been made in the development of techniques for enhancing dose concentration on the tumor while minimizing adverse effects on the normal organs around the affected part. In a treatment using X-rays called IMRT (Intensity Modulated Radiation Therapy), the tumor is irradiated in multiple directions while the collimator shape is being varied. This makes it possible to irradiate the target region of a complicated shape while minimizing the irradiation dose on the surrounding normal tissues. Also utilized is a method that involves rotating a gantry equipped with an irradiation device to irradiate the tumor continuously while the collimator shape is being changed.
In particle therapy, the scanning irradiation method is getting widespread acceptance. This method involves irradiating the inside of the tumor in a manner filling it with fine particle beams so that a high dose is given only to the tumor region. Basically, there is no need for instruments specific to the patient such as the collimator for shaping dose distribution in keeping with the tumor shape; diverse kinds of dose distribution can be formed by this method.
Radiation therapy requires that detailed plans be made beforehand with regard to the irradiation position and tumor status. The irradiation dose and irradiation position are determined in such a manner that the treatment planning system provides a desired does distribution regarding the tumor and its surroundings in advance. Upon advance planning, the conditions in the patient's body are verified most commonly by means of X-ray CT images (called the CT images hereunder). The position of the tumor is designated and the calculations of the dose distribution inside the body are performed very frequently using the CT images.
Although irradiation should preferably be carried out as planned, there can occur errors in practice due to diverse causes. Such errors are attributable not only to the equipment itself and positioning procedures but also to tumor movements under irradiation caused by respiration and heartbeat. Because the tumor movements typically triggered by respiration and heartbeat differ from patient to patient and from one target region to another, they are difficult to evaluate. In IMRT and scanning irradiation, the desired dose distribution is formed by superimposition of complex distribution patterns, so that variations in the distribution induced by respiration and other movements are particularly difficult to predict.
There has been conceived a method aimed at evaluating quantitatively the effects of the tumor movements on the dose distribution by supplementing the CT images with time-varying information to predict movements of the internal organs around the tumor. The CT images furnished with timing information are called four-dimensional (4D) CT images. Starting from an image representing the state of a moving object in a short time range, 4D CT images are formed by a plurality of sets of 3D CT images taken at different points in time.
For example, the ordinary CT image of respiratory movements is an image that time-averages the movements in the respiratory cycle. The method above involves having 3D CT images obtained at various points in time such as in the inhaled state and exhaled state during the respiratory cycle.
There have been several attempts at predicting the effects of tumor movements on the dose distribution calculations using 4D CT images in conjunction with the treatment planning system. For example, JP-2008-80131-A discloses that given a plurality of phases included in 4D CT images, the dose distribution is calculated from the weight of each of the phases. JP-2009-502257-T discloses that given information about the respiratory state recorded during radiation irradiation, one of the phases in the corresponding 4D CT images is selected and the dose distribution is calculated regarding the selected phase.
The dose distribution calculated for each of the phases involved can be integrated by means of the non-rigid registration technique as referenced in the above-cited patent literature. The body structure such as the target region between different phases varies not only in position but also in shape. According to the non-rigid registration technique, with the CT image of a given phase (e.g., exhaled phase) taken as the reference CT image, different points within the target in the reference CT image can be made to correspond to their corresponding positions in the CT images obtained from the other phases. By integrating the doses at these corresponding points, an ultimate dose distribution on the reference CT image can be acquired.
As described above, the dose calculation techniques using 4D CT images are primarily based on calculating the dose distribution on the CT images of various phases and integrating the distributed doses so as to compute the dose distribution for treatment in a moving state expressed by the 4D CT images.